The present invention relates to radio navigation receivers, and in particular to satellite navigation receivers (e.g., a GPS receiver) for precise relative positioning in real time.
Satellite navigation receivers such as GPS and GLONASS are all-weather, worldwide, continuous coverage, satellite-based radio navigation systems. These receivers measure time delays and decode messages from satellites within view of the receivers to determine the information necessary to complete position and time bias calculations. For a detailed discussion see "The Global Positioning System and Inertial Navigation", J. Farrell and M. Barth, McGraw-Hill, 1999.
The quality of the position estimates available to the users of GPS/GLONASS can vary widely. The position errors for different users can range from centimeters to tens of meters. The performance specifications for civil use of the GPS system are given in terms of 95th and 99.99th percentile points of the error in the estimate of all users (i.e., civilian and military). These specifications for the horizontal error are 100 meters and 300 meters respectively for civilian use. Additional resources are required to obtain position estimates of better quality. Real-time position estimates with errors no worse than a few meters can be obtained if the user subscribes to a commercial service broadcasting differential corrections to be applied to the measurements. Actually, in coastal areas such corrections are available for free from the U.S. Coast Guard, which broadcasts them on maritime radio beacons. All the user needs is a radio beacon receiver and a differential-ready GPS receiver.
To get position estimates with centimeter(cm)-level accuracy requires a different approach. Until now, requirements for such accuracy have typically been limited to the geodetic community (e.g., studying plate tectonics), since achieving this level of accuracy has generally required minutes or hours of computation. As is known, GPS carrier phase measurements can provide cm-level positioning accuracy if the integer ambiguities are resolved correctly, and the process of integer ambiguity resolution is often referred to as initialization.
The approach commonly used for initialization (static and kinematic), is to filter measurements from multiple epochs until the floating estimates of the integer ambiguities appear to converge to integer values. With dual-frequency GPS measurements, the wide-lane formulation accelerates this process, but it still takes several minutes. The approach offers robustness in terms of overcoming measurement errors over short stretches. However, this approach is impractical for navigation where a user is unlikely to have the luxury of waiting for minutes for the integer ambiguity to be resolved.
Therefore, there is a need for system that can provide a robust navigation receiver capable of providing centimeter-level accuracy in real-time.